1. Field of the Invention
The present invention generally relates to a half mirror display device for instruments, and particularly, it relates to a half mirror display device for instruments with an improved illumination system.
2. Description of Relevant Art
FIG. 1 shows a view through a display window of a conventional half mirror display device applied to an instrument panel of an automobile, and FIG. 2, a section along line A--A of FIG. 1.
As illustrated in FIG. 1, the display window displays a speedometer 1, a tachometer 3, a fuel gauge 5, a temperature gauge 7 and a shift lever position indicator 9, to an unshown driver or passenger in the automobile.
As shown in FIG. 2, the display window is defined as a front opening of a light shielding, enclosure of a panel structure that comprises a bottom panel 11, a rear panel 25, a top panel 27, unshown lateral panels and necessary support members or elements. The enclosure accommodates therein an optical display system comprising a figure defining system for defining a set of figure elements to be optically observed, a figure illuminating system for illuminating a plurality of figure elements to provide a plurality of corresponding optical monochromatic or multi-chromatic figure elements to be visible, and a semi-transparent mirror member 13 called "half mirror", which is set in an oblique or 45-dgree inclined position relative to a view direction B, with upper and lower edges held by an upper rear support element 13a and a lower front support element 13b, respectively.
The figure defining system comprises a non-transparent background plate 15 formed with a free hole 15a, a needle-shaped light-transmittable pointer 17 rotatable with a support shaft 17a provided through the free hole 15a and engaged with a cotrollable actuator 17b, and a non-transparent design plate 19 provided with various decorative fancy and/or necessary design elements (hereafter collectively "design elements") such as combinations of slits for defining characters, divisions, markings, etc.
The figure illuminating system comprises: a pair of rear cold cathode tubes 29 as a rear light source for indirectly illuminating the pointer 17; a rear light transmitting plate 21 attached to a rear side of the background plate 15 in opposition to the cold cathode tubes 29 and formed with a hole 21a having a circumference tapered to be smaller in diameter at its front end than at its rear end and than the free hole 15a so that the light transmitting plate 21 has about a front end circumference of the tapered hole 21a a peripheral surface region exposed to a base end part of the pointer 17 to be illuminated; a cap 17c for covering the base end part of the pointer 17 and an exposed inner region of the free hole 15a and for defining limits of movement of the pointer 17; a pair of top cold cathode tubes 30 as a top light source for indirectly illuminating the design elements of the design plate 19; and a top light transmitting plate 23 attached to an upper side of the design plate 19 in opposition to the cold cathode tubes 30.
For observation in the view direction B, a luminous image of the pointer 17 Appears as a mobile real image through the half mirror 13, and a set of illuminated images of the design elements of the design plate 19 appears as a set of stationary virtual images on an identical imaginary plane P, as it is reflected on the half mirror 13. The real and virtual images are combined in a spatially overlapping manner, as illustrated in FIG. 1.
The rear light source 29 is dedicated for illumination of the pointer 17. The top light source 30 is dedicated for illumination of the design elements of the design plate
There is an everlasting demand for a light weight design.